**2.4 Initial and boundary conditions**

### *2.4.1 Upper surface dynamic boundary condition*

In this chapter, the data in the *Special Meteorological Data Set for the Analysis of the Thermal Environment of Buildings in China* are adopted to simulate the hourly meteorological data of the typical meteorological year of Chongqing, as shown in **Figure 4**, and the dynamic boundary conditions of the upper surface are written into ANSYS\_FLUENT software through user-defined function for simulation.

#### *2.4.2 Initial soil temperature condition*

Because HGHE is generally shallow buried depth, shallow soil initial temperature distribution is often uneven due to external influence. Based on this, in order to make the simulation more suitable to the actual project, this chapter selects the soil temperature distribution of the hottest month in Chongqing as the initial temperature condition of the simulation, as shown in **Figure 5**. Also, the initial soil temperature conditions are written into ANSYS\_FLUENT software through user-defined function for simulation calculation.

#### *2.4.3 Parameter of soil*

The research background of this chapter is in Chongqing, China, and the soil type widely distributed in Chongqing is strongly weathered mudstone, the specific parameters of which are determined by experiment, as shown in **Table 3**.

**Figure 4.** *Hourly meteorological parameters in Chongqing of the typical year.*

#### **Figure 5.**

*Initial soil temperature distribution in the depth direction in the hottest month of Chongqing.*


#### **Table 3.**

*Thermophysical parameters of strongly weathered mudstone.*

#### **2.5 CFD simulation setup**

The HGHE model in this chapter is a three-dimensional model, so the option of 3D model should be selected on the initiator interface. In the calculation of the threedimensional model, ANSYS\_FLUENT provides two different calculation methods, and the two different calculation methods have their own suitable situations. The calculation method of single precision is rough, but the amount of calculation is relatively small, and it is applied to the case that the model is regular. The calculation method of double precision is mainly used for the situation that the length and size of the fluid domain is large, the fluid domain has more than one part, and each domain is connected by the small size pipeline with drastic temperature changes and high thermal conductivity. Therefore, this chapter chooses to use the double-precision solver to solve the problem.

This chapter mainly simulates the flow characteristics of fluid in HGHE, which belongs to the range of incompressible flow fluid, so the uncoupled implicit solution is adopted. Because the heat transfer process in the buried pipe was ongoing all the time and the data fluctuated within the range, the transient computing mode was selected. At the same time, the energy equation option is opened, the *k* � *ε* flow model is adopted, and the near-wall function is modified at the same time.
